A battery is a device which can take out chemical energy as electric energy utilizing oxidation-reduction reactions occurring on a positive electrode and a negative electrode or can store electric energy as chemical energy, and is utilized as a power source in various equipment.
Recently, along with the rapid spread of mobile electronic equipment, there have been increased needs for a lightweight battery with a large capacity. In order to satisfy the needs, batteries, which utilize alkali metal whose mass per unit electric charge is light, have been developed. Among these batteries, a secondary battery using lithium ions as a carrier is particularly utilized for various mobile equipment for its superiority in charge and discharge characteristics and large capacity.
However, in such lithium-ion secondary battery, when the battery is overcharged due to failures in the power supply circuit and battery charger of electronic equipment or misuse of electronic equipment, excess lithium ions are released from the positive electrode, which sometimes causes thermal instability in active materials in the positive electrode. When being thermally instable, the positive electrode undergoes an abrupt exothermic reaction with an electrolytic solution. If the exothermic reaction proceeds to a considerable extent, it may leads to damage to the battery or the ignition of the battery. Consequently, it is important issues to prevent the battery from being overcharged and assure safety.
For the purpose of ensuring safety on the occasion when the battery is overcharged, there have been proposed protective devices or elements, such as a polyswitch, a PTC element, a temperature fuse, a current fuse, etc., a overcharge prevention circuit and the like. The use of the protective element or the prevention circuit, however, imposes significant limits on the miniaturization of a battery pack and cost savings. Therefore, the safety of the battery should preferably be secured without using the protective element or the prevention circuit.
To solve the problem, it has been proposed that the battery contain such additives as to improve safety at the time of overcharging the battery.
For example, Japanese Patent Application laid open No. HEI7-302614 and Japanese Patent Application laid open No. HEI9-50822 have disclosed a nonaqueous electrolyte second battery using metal material consisting mainly of lithium or carbon material capable of doping/undoping lithium for the negative electrode, and lithium and an complex oxide of transition metals for the positive electrode, in which a nonaqueous electrolytic solution contains a benzenes compound (anisole derivative(s), xylene derivative(s), etc.) or a biphenyl compound (dimethoxy-biphenyl derivative(s)) having a molecular weight of 500 or less, with a π orbital electron that exhibits a reversible redox (oxidation-reduction) potential more noble than the potential at the positive electrode of the fully charged battery. According to the patent applications, with the addition of such aromatic compounds to the electrolytic solution, an overcharge reaction or the excessive release of lithium from the positive electrode is prevented, which improves the safety and reliability of the battery.
Additionally, Japanese Patent Application laid open No. HEI9-106835 has disclosed a nonaqueous lithium battery using a lithium adjected compound for the positive electrode and a carbonaceous compound for the negative electrode, in which an electrolytic solution contains aromatic additives such as biphenyl, 3-chloro-thiophene, furan or the like. From its description, these additives are polymerized at a battery voltage higher than the maximum operating voltage of the battery, which increases the inner voltage of the battery, and, therefore, protects the battery in an overcharged state.
Besides, Japanese Patent Application laid open No. 2001-15155 has disclosed an electrolytic solution for a lithium secondary battery obtained by dissolving the lithium salt in an organic solvent, in which a phenyl group contains an alkylbenzene derivative(s) or a cycloalkylbenzene derivative(s) having adjacent tertiary carbon. These additives are polymerized under the condition of overcharge, and generate a polymer that is unlikely to remelt. The polymer is said to act as a resistor or a resistive element. In addition, there is described in the patent application a battery with an electric current breaker that interrupts charging when the gas pressure inside the battery is equal to or more than a prescribed value. It is alleged that hydrogen gas is exhaled due to decomposition of the additives when the battery is overcharged, which activates the electric current breaker to prevent the overcharge.